The present invention relates generally to earth moving equipment, and more particularly to semiautomatic control of earthmoving machines based on attitude measurement.
Various construction equipment is used for performing construction projects, such as airports and roads. These projects typically involve preparation of land according to architectural and engineering specifications. Earthmoving machines, such as bulldozers and graders, are used to prepare the site. Skilled operators can control these machines to perform high-quality grading operations to prepare the site for final use or to prepare the site for further work (such as adding road ballast, pouring concrete, or paving with asphalt). In a construction project, surveyors typically do an initial layout of a jobsite (for example, set the desired boundaries and height levels) and perform additional layouts as the construction works proceed. A layout is typically setup with visual markers, such as stakes and poles, which may be viewed by a machine operator. This procedure is very time-consuming, especially when high accuracy of the terrain (ground) profile is required. Multiple iterations of setting up a layout and checking the terrain profile are often required.
To attain a precise terrain profile, the machine operator needs to be highly qualified and experienced. Not only does he need to adjust the implement (such as a blade) position according to the height assigned by the markers, but he must also compensate for parasitic effects, such as perturbation factors from the underlying terrain and blade load on the machine body, that tend to arbitrarily change the spatial position of the blade. Furthermore, simultaneous dual-channel adjustment of blade position with respect to height (elevation) and degree of inclination (slope) is a difficult operation. Weather conditions may also adversely impact attainment of the required terrain profile, since limited visibility often prevents the operator from observing the markers.
To assist the operator in attaining the required terrain profile, different types of grading control systems may be installed on the machines. These grading control systems use sensor measurements to position the implement according to the assigned terrain profile. The sensors are mounted onto the machine itself and do not require visual observations of the blade position relative to any markers. Grading control systems may be divided into two major categories: indicator and automatic. The indicator systems provide the operator with visual mismatch indicators representing the error between the actual and desired positions of the implement, according to a set of user-defined coordinates. The operator visually observes the indicators in the machine cab and makes appropriate adjustments by manually activating a control lever which controls the blade hydraulic cylinders. The automatic systems may directly control the blade hydraulic cylinders based on error signals. Electronically controlled valves are used in such systems. Automatic systems are more expensive than indicator ones since additional components are needed for automatic hydraulic control.
Different indicator systems accommodate different degrees of freedom in the implement positioning system. To unambiguously determine the position and orientation of a ground-based object (such as an implement on an earthmoving machine), three position coordinates (for instance, geographic latitude, longitude, and height) and three attitude angles (for example, pitch, roll, and heading) are needed; that is, six degrees of freedom. Some applications, however, may use systems with fewer than six degrees of freedom.
A system with one angular degree of freedom may be used for estimating a blade roll or machine body roll angle with respect to the horizon. It can be based on liquid or Micro-Electro-Mechanical Systems (MEMS) accelerometer sensors sensitive to the Earth's gravitational field. To measure roll angle, the sensitive axis of the sensor is placed along the lateral direction. Such sensors are called inertial because they operate within an inertial coordinate frame obeying Newton's laws of motion. Adding another sensor, such as a longitudinal gravitational sensor, allows the system to also measure a pitch angle, thereby providing estimation of two angular degrees of freedom. U.S. Pat. No. 4,561,188 discusses an example of a indicator system for two degrees of freedom. U.S. Pat. No. 7,121,355 discusses an example of an automatic system for two degrees of freedom.
Angle-measuring systems are not limited to determining and controlling machine attitude. They also help form a desired height profile (for example, a flat horizontal profile is attained by keeping both the pitch angle and the roll angle constant during grading). Such systems, however, provide low-accuracy grading since they are insensitive to blade-height variations (errors), such as those arising from the factors discussed above. Inaccuracies also arise from the gravitational sensors themselves, since they are sensitive to dynamic accelerations caused by machine motion. These inaccuracies are particularly significant for longitudinal sensors, since dynamic acceleration is maximal along the longitudinal axis. Due to the accumulation of height errors, the actual profile can differ in height from the desired profile by a considerable value (at least tens of cm), especially for sites which span a long distance. Inertial sensors alone may not be sufficient to detect changes in the height profile. For example, the bulldozer angular position at a local point may remain fixed at the same value as the one at the initial setting (for example, it may have been set to horizontal at the beginning of the swath) but the height can be considerably different from the initial one.
To enhance the system operability, a number of height-measuring sensors can be added. U.S. Pat. No. 5,917,593, for example, discusses a system including a mast with a vertical linear photocell array installed on the blade. The array receives signals from a stationary laser transmitter (base station), which transmits a narrow laser beam rotating at a constant speed. The rotation axis is perpendicular to the axis of the laser beam. A laser plane is thereby formed in space which can be oriented horizontally or at an angle to the ground surface. By determining the number of the photocell receiving the laser beam at the current instant, the blade height with respect to the laser transmitter may be estimated. If a gravitational sensor is added to measure a roll angle, then two degrees of freedom [one linear (height) and one angular (roll)] can be determined and controlled, and the accumulated height error can be efficiently eliminated for the desired profile set as a plane. The main drawback of such systems is the inability to form complex profiles differing from a simple plane. Also, the range of operation is usually limited to a few hundreds of meters. To generate zig-zag planes, the slopes and positions of the transmitter must be changed. This process is inconvenient in practice.
Instead of a photocell array, systems for forming complex profiles may be equipped with a Global Navigation Satellite System (GNSS) receiver. Examples are discussed in US Patent Application Publication No. 2009/0069987 and U.S. Pat. No. 7,317,977. Another approach uses an optical prism, whose position is determined by a stationary laser robotic total station fixed on a construction site within a line-of-sight distance. Such a system can include a roll sensor or two or more GNSS receivers to estimate attitude. These systems can be also fitted with electronically controlled valves, which allow automation of the blade-drive process. Estimating six degrees of freedom enable attainment of centimeter-level accuracy for forming the complex terrain profile. The drawback of such systems is their high cost (up to hundreds of thousands US dollars) and the necessity of installing and managing a base station (a GNSS receiver with a modem to transmit differential corrections to the machine control board or a laser robotic total station).
Angular control systems with two degrees of freedom may be produced at low cost, and they may operate in a fully autonomous mode, without a base station. As discussed above, standard angular control systems have limitations with respect to attaining high-accuracy terrain profiles. What are needed are methods and apparatus for reducing height errors in angular control systems with two degrees of freedom.